Yes, app_fee_money works in Sandbox however it won’t create a balance that will show in the Sandbox Seller Dashboard. If the app_fee_money shows on the payment you successfully took a payment that collected app_fee_money 
Yes, app_fee_money works in Sandbox however it won’t create a balance that will show in the Sandbox Seller Dashboard. If the app_fee_money shows on the payment you successfully took a payment that collected app_fee_money